Semiconductor nanocrystals with small diameters can have properties intermediate between molecular and bulk forms of matter. Small diameter semiconductor materials can exhibit quantum confinement of both the electron and hole in three dimensions. Quantum confinement plays a key role in determining the size-dependent optical properties of semiconductor nanocrystals. One effect of quantum confinement is an increase in the effective band gap of the material with decreasing nanocrystal size. As the size of the semiconductor nanocrystal decreases, both the optical absorption and emission of the nanocrystals shift to higher energy (i.e., to the blue). The extinction coefficient is also size-dependent. As the size of a nanocrystal increases, the extinction coefficient of the particle increases proportionally. Consequently, for a given material, larger semiconductor nanocrystals (e.g., nanocrystals emitting in the red or IR spectral region) are typically brighter than smaller nanocrystals (e.g., nanocrystals emitting at wavelengths below the red spectral region).
Nanocrystals frequently include a semiconductor core and a passivating, semiconductor shell. Shell materials with bandgaps higher than those of the core materials can minimize deep-trap emission sites and can enhance quantum yield (QY) and stability of the nanocrystal particle. The optical properties (e.g., brightness) and stability of the nanocrystal can be further improved by the use of very thick shells. However, a common drawback of using thick shells to enhance the brightness of nanocrystals is an associated shift of the emission band to longer wavelengths (i.e., red-shifting). This can be a problem for applications where both bright materials and multiple colors spanning the visible spectrum (e.g., blue to red) are required. Further, currently available nanocrystals, especially those with lower emission wavelengths, are often insufficiently bright, typically exhibit pronounced levels of intermittent blinking and are not often photostable under prolonged irradiation.
Thus, there is a need to develop approaches to provide bright and stable nanocrystals having high QY and/or high extinction coefficients. A need also exists for bright and stable semiconductor nanocrystals with high energy emission in the visible or UV regions of the electromagnetic spectrum (e.g., blue, green, yellow, and orange). A further need exists to provide small (such that they are useful in fluorescence resonance energy transfer or cell nucleus staining applications) and stable nanocrystals which address the problems posed by nanocrystal fluorescent intermittency (as this intermittency complicates the reliable use of “blinking” nanocrystals as a single photon light source for quantum informatics and as biolabels for real-time monitoring of single biomolecules).